Types of Olfactory Disorder

Olfactory Loss

Smell loss is the most common type of olfactory disorder. Anosmia is the medical term for the absence of the sense of smell, whilst hyposmia refers to a reduced sense of smell. The point at which hyposmia becomes anosmia and vice versa isn’t exactly clear, however. There are many degrees of olfactory loss; some people lose it completely and suddenly, whilst others experience it gradually over time, sometimes not realising that it is deteriorating. Some people are born without a sense of smell – this is known as congenital anosmia.

Anosmia is regarded as being a rare condition, but is probably more common than many people think. Various studies undertaken in a number of countries to try to establish the prevalence of smell loss have suggested that anywhere from 0.1% to over 5% of people are anosmic with up to 50% being hyposmic. Results will of course vary according to who was surveyed in each study, but taking all this into account it is perhaps reasonable to say that potentially around 5% of the population has anosmia or severe hyposmia.

Qualitative Olfactory Disorders

These are usually side-effects of olfactory loss rather than conditions in their own right. Parosmia refers to distortions of the sense of smell, where an odour can be perceived but it doesn’t smell the way it should (usually unpleasant). Phantosmia, as the name suggests, refers to ‘phantom’ smells; being able to detect an odour when there is no actual smell present.

Causes of Olfactory Disorders

Sinonasal disorders and allergies – 70%*

This group of conditions are responsible for what clinicians call ‘conductive olfactory loss’; obstruction in the nose that prevents odourant molecules reaching the olfactory receptor neurones. The following conditions all fall within this category:

Chronic Rhinosinusitis

Deviated septum or foreign body obstructing the flow of air

Allergic rhinitus caused by pollen, dust, animal hair etc

Post-viral olfactory loss (PVOL) 11%*

It’s probably safe to say that everyone has lost their sense of smell for short periods during their life when they’ve had a cold or flu. For some people, however, the symptoms of the cold clear but the sense of smell does not return. The various viruses that cause the common cold and flu can damage and and interfere with the olfactory epithelium, the lining at top of the nose that contains the olfactory receptor cells.

Post-traumatic olfactory loss (PTOL) 5%*

Head injuries can result in the loss of the sense of smell. The extent of this loss can depend not not only on severity of the injury but also the part of the head damaged. Injury to the back or sides of the head is more likely to result in a loss of smell, as the impact can cause the brain to collide with the inside front of the skull. This can cause damage not only to the brain itself but to other parts of the olfactory system such as the olfactory nerve.

Idiopathic anosmia 6%*

This refers to patients in whom after extensive testing no cause for the loss of sense of smell is found. It is important that this diagnosis is made only after blood tests and brain scans have been performed.

Congenital anosmia 1%*

Occasionally people are born with no working sense of smell. Why this happens is not well understood. Sometimes this may be part of a condition called Kallmann syndrome which includes a lack of hormone production in the pituitary gland due to a defect in the part of the brain called the hypothalamus. Some congenital anosmics may however have the loss of smell in isolation. There are also people who may believe they have had no sense of smell from birth but have perhaps lost it at a young age due to a virus or bang to the head.